Disorders of lipid metabolism (dyslipidemias) include various conditions characterized by abnormal concentrations of one or more lipids (i.e. cholesterol and triglycerides), and/or apolipoproteins (i.e., apolipoproteins A, B, C and E), and/or lipoproteins (i.e., the macromolecular complexes formed by the lipid and the apolipoprotein that allow lipids to circulate in blood, such as Low Density Lipoproteins (LDL), Very Low Density Lipoproteins (VLDL) and Intermediate Density Lipoproteins (IDL)). Cholesterol is mostly carried in Low Density Lipoproteins (LDL), and this component is commonly known as the “bad” cholesterol because it has been shown that elevations in LDL-cholesterol correlate closely to the risk of coronary heart disease. A smaller component of cholesterol is carried in the High Density Lipoproteins (ODL) and is commonly known as the “good” cholesterol. In fact, it is known that the primary function of HDL is to accept cholesterol deposited in the arterial wall and to transport it back to the liver for disposal through the intestine. Although it is desirable to lower elevated levels of LDL cholesterol, it is also desirable to increase levels of HDL cholesterol. Generally, it has been found that increased levels of HDL are associated with a lower risk for coronary heart disease (CHD). See, for example, Gordon, et al., Am. J. Med., 62, 707-714 (1977); Stampfer, et al., N. England J. Med., 325, 373-381 (1991); and Kannel, et al., Ann. Internal Med., 90, 85-91 (1979). An example of an HDL raising agent is nicotinic acid, a drug with limited utility because doses that achieve HDL raising are associated with undesirable effects, such as flushing.
Dyslipidemias were originally classified by Fredrickson according to the combination of alterations mentioned above. The Fredrickson classification includes 6 phenotypes (i.e., I, IIa, IIb, III, IV and V) with the most common being the isolated hypercholesterolemia (or type IIa) which is usually accompanied by elevated concentrations of total and LDL cholesterol. The initial treatment for hypercholesterolemia is often to modify the diet to one low in fat and cholesterol, coupled with appropriate physical exercise, followed by drug therapy when LDL-lowering goals are not met by diet and exercise alone.
A second common form of dyslipidemia is the mixed or combined hyperlipidemia or type IIb and III of the Fredrickson classification. This dyslipidemia is often prevalent in patients with type 2 diabetes, obesity and the metabolic syndrome. In this dyslipidemia there are modest elevations of LDL-cholesterol, accompanied by more pronounced elevations of small dense LDL-cholesterol particles, VLDL and/or IDL (i.e., triglyceride rich lipoproteins), and total triglycerides. In addition, concentrations of HDL are often low.
Peroxisome proliferators are a structurally diverse group of compounds that when administered to rodents elicit dramatic increases in the size and number of hepatic and renal peroxisomes, as well as concomitant increases in the capacity of peroxisomes to metabolize fatty acids via increased expression of the enzymes of the beta-oxidation cycle. Compounds of this group include but are not limited to the fibrate class of lipid modulating drugs, herbicides, phthalate plasticizers and the glitazones, a class of compounds that has been under investigation for the treatment of type 2 diabetes. Peroxisome proliferation is also triggered by dietary or physiological factors such as a high-fat diet and cold acclimatization.
Three sub-types of peroxisome proliferator activated receptor (PPAR) have been discovered and described; they are peroxisome proliferator activated receptor alpha (PPARα), peroxisome proliferator activated receptor gamma (PPARγ) and peroxisome proliferator activated receptor delta (PPARδ). PPARα is activated by a number of medium and long-chain fatty acids, and it is involved in stimulating β-oxidation of fatty acids. PPARα is also associated with the activity of fibrates and fatty acids in rodents and humans. Fibric acid derivatives such as clofibrate, fenofibrate, bezafibrate, ciprofibrate, beclofibrate and etofibrate, as well as gemfibrozil, each of which are PPARα ligands and/or activators, produce a substantial reduction in plasma triglycerides as well as some increase in HDL. The effects on LDL cholesterol are inconsistent and might depend upon the compound and/or the dyslipidemic phenotype. For these reasons, this class of compounds has been primarily used to treat hypertriglyceridemia (i.e, Fredrickson Type IV and V) and/or mixed hyperlipidemia.
The PPARγ receptor subtypes are involved in activating the program of adipocyte differentiation and are not involved in stimulating peroxisome proliferation in the liver. There are two known protein isoforms of PPARγ: PPARγ1 and PPARγ2 which differ only in that PPARγ2 contains an additional 28 amino acids present at the amino terminus. The DNA sequences for the human isotypes are described in Elbrecht, et al., BBRC 224;431-437 (1996). In mice, PPARγ2 is expressed specifically in fat cells. Tontonoz et al., Cell 79: 1147-1156 (1994) provide evidence to show that one physiological role of PPARγ2 is to induce adipocyte differentiation. As with other members of the nuclear hormone receptor superfamily, PPARγ2 regulates the expression of genes through interaction with other proteins and binding to hormone response elements, for example in the 5′ flanking regions of responsive genes. An example of a PPARγ2 responsive gene is the tissue-specific adipocyte P2 gene. Although peroxisome proliferators, including the fibrates and fatty acids, activate the transcriptional activity of PPAR's, only prostaglandin J2 derivatives have been identified as potential natural ligands of the PPARγ subtype, which also binds thiazolidinedione antidiabetic agents with high affinity.
The human nuclear receptor gene PPARδ (hPPARδ) has been cloned from a human osteosarcoma cell cDNA library and is fully described in A. Schmidt et al., Molecular Endocrinology, 6:1634-1641 (1992). It should be noted that PPARδ is also referred to in the literature as PPARβ and as NUC1, and each of these names refers to the same receptor; in Schmidt et al. the receptor is referred to as NUC1.
In WO96/01430, a human PPAR subtype, hNUC1B, is disclosed. The amino acid sequence of hNUC1B differs from human PPARδ (referred to therein as hNUC1) by one amino acid, i.e., alanine at position 292. Based on in vivo experiments described therein, the authors suggest that hNUC1B protein represses hPPARα and thyroid hormone receptor protein activity.
It has been disclosed in WO97/28149 that agonists of PPARδ are useful in raising HDL plasma levels. PPARδ agonists have recently been disclosed in U.S. Provisional Application Ser. No. 60/297,356 as having utility in the treatment of various inflammatory diseases, such as rheumatoid arthritis. WO97/27857, 97/28115, 97/28137 and 97/27847 disclose compounds that are useful as antidiabetic, antiobesity, anti-atherosclerosis and antihyperlipidemic agents, and which activate PPARs.
It is generally believed that glitazones exert their effects by binding to the peroxisome proliferator activated receptor (PPAR) family of receptors, controlling certain transcription elements having to do with the biological entities listed above. Glitazones are benzyl-2,4-thiazolidinedione derivatives. See Hulin et al., Current Pharm. Design (1996) 2, 85-102.
A number of glitazones that are PPAR agonists have been approved for use in the treatment of diabetes. These include troglitazone, rosiglitazone and pioglitazone, all of which are primarily or exclusively PPARγ agonists. Many of the newer PPAR agonists that are currently under development or are in clinical trials have dual PPARα and γ activity, such as KRP-297. The PPARα/γ agonists are expected to improve both insulin sensitivity and the lipid profile in patients having NIDDM.
Although glitazones have been beneficial in the treatment of NIDDM, there have been some serious adverse events associated with the use of the compounds, especially troglitazone, which was eventually withdrawn. The most serious adverse events have been liver toxicity, which resulted in a number of deaths. Because of the problems that have occurred with the glitazones, researchers in a number of laboratories have been investigating classes of PPAR agonists that do not contain 1,3-thiazolidinedione moieties and therefore are not glitazones.
Compounds that are agonists of the various PPAR sub-types are expected to be useful in the treatment of diseases and conditions that respond to treatment with PPAR agonists, regardless of whether the compounds are glitazones. These include dyslipidemia, diabetes, and related conditions. PPARα agonists improve the lipid profile and alleviate dyslipidemias by reducing elevated LDL levels, reducing elevated triglyceride levels, and increasing HDL levels. PPARγ agonists improve insulin sensitivity, reducing the need for insulin secretagogues and insulin injections in patients with NIDDM. The role of PPARδ is less well defined, but PPARδ also appears to help control hyperlipidemia and hyperglycemia in type 2 diabetic patients.